The present invention relates to a lithographic printing method and device.
Generally, the lithographic press prints in such a way that a printing plate is wound on a plate cylinder while being mechanically fastened thereonto. A lithographic printing plate uses a support of a material other than metal (e.g., plastic film, paper or the like), which is easy to handle. However, it is not satisfactory in its dimensional stability. For example, friction between a blanket cylinder and a form plate causes a partial strain resulting in the deformation of the form plate, thereby degrading the printing dimensions and the accuracy of the printing positions relative to a printing paper.
The form plate using a support made of such a material has been used in a limited type of printing, e.g., a simple printing for a small number of prints, which does not require a high registration accuracy. Some modification or alteration needs to be made on the form plate when it is used for standard printing, e.g., high grade printing of multi-color, large scale printing machines or the like.
To cope with this problem, the Japanese Utility Model Unexamined Publication No. Sho. 58-1046 suggested that a press plate should be directly bonded to a preparatory form plate or form cylinder by use of a double-coated adhesive sheet or by use of a spray adhesive of acrylic or rubber material.
This method firmly bonds the printing plate to the plate cylinder. The firm bonding by this method does not allow fine position adjustment of the printing plate on the plate cylinder, and makes it extremely difficult to remove the form plate after printing.
A plate-forming and printing method based on a CTP (computer to plate), which gradually increases its presence in the market, is superior to the conventional plate-forming and printing method (whose exposure step/process for the press plate is based on a close-contact exposure using a lith film), in that the size and position accuracy of an image (exposure) to the press plate is good, and the registration is easy in multi-color printing. However, the lithographic printing plate using the support of a plastic film, a paper or the like, fails to fully utilize the advantage of easy registration in multi-color printing by the CTP because of its disadvantage referred to above.
Accordingly, an object of the present invention is to provide a printing method using a support of a plastic film, a paper or the like, which has the following advantageous features of: 1) solving the problem of the form plate being partially deformed, thereby degrading the printing dimensions and an accuracy of printing positions relative to a printing paper, 2) enabling the registration to be easy in the multi-color printing in the CTP-basis plate-forming/printing method, and 3) solving the problem of poor working efficiency which is the problem in printing in the conventional printing method.
The above object can be achieved by a printing method, according to a first aspect of the present invention, for performing a lithographic printing in a multiple of colors by use of flexible press plates, in a lithographic printing device including plate cylinders provided for the respective colors wherein each of the plate cylinders includes a series of protruded pieces formed on an outer peripheral surface and aligned in the circumferential direction thereof, the printing method including the steps of:
forming a series of engaging holes on each of the press plates, the engaging holes being aligned in a printing direction in such a manner that the protruded pieces are insertable into the engaging holes;
attaching the press plate to the plate cylinder while the engaging holes of the press plate are engaged with the protruded pieces of each of the plate cylinders; and
carrying out the printing,
wherein the engaging holes forming step is conducted while a following equation is satisfied:
2Dxe2x89xa6Pxe2x89xa64.06xc3x97103xc3x97D/W
xe2x80x83where P defines at least one of a pitch of the series of engaging holes in the printing direction and a pitch of the series of protruded pieces in the circumferential direction, D indicates a diameter of each protruded piece, and W denotes a width of the press plate.
In the above-mentioned printing method, it is preferable that the printing step is conducted while rotating the plate cylinder in a state that the engaging holes of the press plate are being engaged with the protruded pieces.
In the above-mentioned printing method, it is advantageous that the lithographic printing device further includes an exposure drum which has a series of protruded pieces formed on and along the outer circumferential surface thereof so as to be inserted into the engaging holes, and the printing method further includes the steps of:
attaching the press plate to the exposure drum while the engaging holes of the press plate are engaged with the protruded pieces of the exposure drum; and
exposing the press plate.
In addition, the above-mentioned object can also be attained by a printing method, according to a second aspect of the present invention, for performing a lithographic printing in a multiple of colors by use of flexible press plates, in a lithographic printing device including plate cylinders provided for the respective colors, wherein each of the plate cylinders includes a series of protruded pieces formed on an outer peripheral surface and aligned in the circumferential direction thereof, the printing method including the steps of:
drawing out a press plate in the form of a roll which is stored in a form-plate forming device;
cutting the press plate thus drawn out to form a sheet-like press plate;
forming a series of engaging holes on each of the press plates, the engaging holes being aligned in a printing direction;
attaching the press plate to the plate cylinder while the engaging holes of the press plate are engaged with the protruded pieces of each the plate cylinder; and
carrying out the printing.
In the above-mentioned printing method, it is preferable that the lithographic printing device further comprises an exposure drum which has series of protruded pieces formed on and along the outer circumferential surface thereof so as to be inserted into the engaging holes of the press plate, and
the printing method further includes the steps of:
attaching the press plate to the exposure drum while the engaging holes of the press plate are engaged with the protruded pieces of the exposure drum; and
exposing the press plate.
Further, in the above-mentioned printing method, it is advantageous that a pitch of the series of engaging holes in the printing direction is determined so as to satisfy the following equation:
2Dxe2x89xa6Pxe2x89xa64.06xc3x97103xc3x97D/W
where P defines a pitch of the series of engaging holes in the printing direction, D indicates a diameter of each protruded piece, and W denotes a width of the press plate.
Further, the above-mentioned object can also be attained by a lithographic printing device, according to a third aspect of the present invention, including:
a plurality of plate cylinders provided for the respective colors;
a plurality of packing sheet respectively attached to the plate cylinder and each including a series of protruded pieces aligned in a circumferential direction of the respective plate cylinder,
wherein the plate having engaging holes is conveyed in a printing direction while the engaging holes are brought into engagement with the protruded pieces, and
the protruded pieces satisfy a following equation:
2Dxe2x89xa6Pxe2x89xa64.06xc3x97103xc3x97D/W
where P defines a pitch of the series of protruded pieces in the circumferential direction, D indicates a diameter of each protruded piece, and W denotes a width of a press plate.
In the printing method according to the first aspect of the present invention, a strength of the area of the press plate located between the adjacent engaging holes is satisfactory, and durable for a load acting on one engaging hole.
When the pitch of the protruded pieces series is shorter than 2D, that strength is insufficient, and when that pitch is longer than 4.06xc3x97103 D/W, the load acting on one engaging hole is large, so that a deformation amount of one engaging hole exceeds a tolerable value of a deformation amount. Therefore, to secure a reliable holding of the press plate by the plate cylinder, the following relationship needs to be satisfied:
2Dxe2x89xa6Pxe2x89xa64.06xc3x97103xc3x97D/W.
In the printing method according to the second aspect of the present invention, the cutting of a rolled plate material and the formation of engaging holes in the plate material are concurrently or successively carried out. This leads to improvement in working efficiency. Further, the exposure and printing processes are carried out using the same positioning reference holes. This leads to the ease of color registration in the multi-color printing.
In the printing method above, when the lithographic printing device further includes an exposure drum which has a series of protruded pieces formed on and along the outer circumferential surface thereof so as to be inserted into the engaging holes of the press plate, the dimensions of the press plate and the image dimensions relative to the press plate, and the positioning accuracy when the exposure process is carried out, may be exactly the same as those when the printing is carried out. This feature provides a considerably exact registration of colors.
With the packing sheet according to the third aspect of the present invention, there is no need for working the plate cylinder in any way. Therefore, easy color registration is made at low cost.
Note that Japanese Patent Unexamined Publication No. Hei-7-108672 teaches that a series of protruded pieces are formed on and along the outer circumferential surface of a plate cylinder of a gravure printing press. Engaging holes of the press plate are applied to the protruded pieces of the plate cylinder, to thereby position the press plate to the plate cylinder. The engaging holes and the protruded pieces are used for causing the plate cylinder to hold the press plate in a tension state. Therefore, the disclosed technique is not concerned with a technique to firmly hold the press plate without any shift even under high printing pressure as in offset printing. No description on an implant pitch of the protruded pieces series is found in the publication. The technique of the publication is directed to the sheet-like press plate after cut, and hence is not relevant to the rolled plate material.